As known, all connections between subscriber station equipment in a common telephone system is provided through an interconnect, or matrix switch. In early telephone systems the switch provided a "hard wire" interconnect between station equipment. Later systems provided increased interconnect capability by the shared use, e.g. "spaced division multiplexing" of common wire interconnections, or by pulse code modulation (PCM) of the information coupled with time division multiplexing (TDM) of the interconnects. Nearly all contemporary systems use some type of TDM switch in which each off-hook station subscriber is allocated a specific periodic time interval for information transfer through the switch to the interconnected party subscriber(s).
The TDM type switch has N number of time slots per PCM sample time interval per M number of input/output (I/O) lines to provide a simultaneous M X N number of interconnects. If this provides simultaneous servicing of 100% of the system subscribers the switch is "nonblocking", i.e. no subscriber wait time. The number of TDM time slots per PCM sample time is limited by the telephone sample frequency, typically eight KHz.
Typical of the early TDM stored program system switches is that disclosed in U.S. Pat. No. 3,268,669 by Vigliante et al and assigned to Bell Telephone Laboratories, Inc. The switch regulated by a central control, provides subscriber equipment interconnect in one of fifty TDM signal sample time slots in each periodic signal sample frame. The subscribers compete for the fifty available call slots on a first-come first-serve queue and each time slot remained associated with the party subscriber's station equipment for the duration of the interconnect. Only an on-hook condition would release the time slot for use by the next in line off-hook subscriber.
A higher efficiency interconnect switch is disclosed by Regnier et al in U.S. Pat. No. 3,851,105, which improves the Vigliante et al system through a combined TDM and space division multiplexed scheme in which dual memories including a speech memory and a control memory temporarily store subscriber PCM information samples prior to, and after, (separate input and an output stage memories) the space division multiplexing of the individual samples through a space division switch interconnect. The memory interface reduces the switching overhead time by stacking the samples for presentation to the space division switch, thereby improving overall efficiency by lowering interconnect time per sample. Providing memory storage density which is twice the space division interconnect capacity provides a nonblocking characteristic. It does, however, require a combination of both TDM and space division multiplexing hardware.
U.S. Pat. No. 3,908,092 to Hight et al also discloses an improvement to the Vigliante et al system by reducing the central supervisory control overhead. Call processing is streamlined by distributed processing which occurs at a higher 15.625 KHz sample frequency. The system provides for 64 call slots per switch with 65 time slots per sampling frame, such that successive frames begin with the next higher slot number in a frame modulo 64 representing a call processing cycle. In operation, call processing occurs at the coincidence of a first call slot number with the same frame number. If the call slot number is active (off-hook) the call is processed by the system. If the call slot number is idle the system scans the remaining switch ports to determine if they have transitioned to off-hook status; the first discovered off-hook port is assigned to the idle call slot (e.g. FIFO) for the interim of that active state in call processing for that port occurs on each appearance of that number frame.